This invention relates to thyristors and more particularly to an improved structure of high-voltage and large-current thyristors suitable for power controlling.
A thyristor of the type comprising a semiconductor body having between two major surfaces four contiguously laminated layers of alternate different conduction types of pnpn to form at least three p-n junctions between the adjacent two layers, a pair of main electrodes making ohmic contact to the surfaces of two outer layers, and a gate electrode in contact with one intermediate layer serves as a semiconductor switching element which switches the conduction state between the main electrodes from off-state to on-state when a relatively small electrical signal is applied to the gate electrode. In a thyristor of this type, the most important parts for determining the electrical characteristics of the thyristor, such as voltage blocking capability, conduction capability and switching speed, are two intermediate layers, (called base layers) and especially important design parameters are the dimensions, thickness in particular, resistivity and the impurity concentration profile of the base layers. Of the two base layers, one base layer having a higher resistivity has the resistivity and dimension (thickness) which are substantially definitely determined by the intended value of withstand voltage, almost losing the degree of freedom of design contributive to the improvement in the capabilities. From viewpoint of improving the capabilities of a thyristor, discussion must be concentrated on the other base layer having a lower resistivity. The other base layer is usually formed by solid diffusion technique of impurity (dopant) from the surface of semiconductor body.
To follow the preparation process generally employed, description will be given of a thyristor having the starting material of an n-type conduction semiconductor wafer and the other base layer of p-type formed therein by the solid diffusion technique. One base layer of higher resistivity and the other base layer are herein called an n-base layer n.sub.B and a p-base layer p.sub.B, respectively. One outer layer adjoining the p-base layer and the other outer layer adjoining the n-base layer are called an n-emitter layer n.sub.E and a p-emitter layer p.sub.E, respectively.
The resistivity and the impurity concentration profile of the p-base layer directly affect the characteristics required of the thyristor such as withstand capability to critical rate of rise of off-state voltage dv/dt, gate firing sensitivity and the like. Required of a high-voltage thyristor for use in a thyristor valve of an ac-dc converter for dc power transmission are high dv/dt withstand capability of more than 1500 v/.mu.s and a non-firing gate current of more than 10 mA. An excessive gate sensitivity by which the gate is enabled by a small current is unfavorable and the gate sensitivity must be moderate especially at the maximum operating temperature for the purpose of preventing the erroneous firing of thyristor which would otherwise be caused by a very small current induced in the gate circuit. The realization of those high dv/dt capabilities and moderate, rather low gate firing sensitivity can be ensured by decreasing the resistivity of the p-base layer. In the conventional thyristor design, the sheet resistivity and impurity concentration of the p-base layer were mainly determined from these viewpoints.
FIG. 1 shows impurity concentration profiles of n-emitter and p-base layers of a thyristor conventionally used.
As will be seen from an exemplary illustration in FIG. 1, the p-base layer interposed between an n-emitter layer/p-base layer junction J.sub.3 and a p-base layer/n-base layer junction J.sub.2 is usually designed to have a sheet resistance of about 100 to 200 .OMEGA./.quadrature. and an impurity concentration of about 10.sup.6 to 10.sup.17 atoms/cm.sup.3 in the vicinity of the junction J.sub.3. This has been well known as disclosed, for example, in (1) "3000 Volt and 1300 Ampere Two inch Diameter Thyristor" by C. K. Chu et al in Proceedings of the 5th Conference on Solid State Devices, Tokyo, 1973, Suppl. J.J.A.P., vol. 43, 1974 and (2) "Application of New Technologies to H.V.D.C. Thyristor Production" by K. H. Sommer et al in World Electrotechnical Congress Record, section 5A, paper 47.
The conventional thyristor with the aforementioned profile has the following disadvantages.
One defect results from the high impurity concentration in the vicinity of the junction J.sub.3. The p-base layer having the high impurity concentration in the vicinity of the junction J.sub.3 leads to shortening of the lifetime of carriers in the p-base layer and reduction in the emitter injection efficiency. As a result, on-characteristic for high conduction is impaired. The high impurity concentration gradient in the vicinity of the junction J.sub.2 responsible for increase in the surface electric field at a portion of the junction exposed to the thyristor edge introduces the other defect that makes it difficult to achieve the high withstand voltage capability. To eliminate the latter defect, some of the conventional thyristors have been manufactured having a two-step impurity concentration profile as exemplified in FIG. 2. More particularly, this type of thyristor comprises a p-base layer consisting of two different impurity concentration regions having over the wafer a first step region P.sub.B1 of low concentration formed adjacent to the junction J.sub.2 and a second step region P.sub.B2 of relatively high concentration formed adjacent to the junction J.sub.3, as disclosed, for example in the aforementioned literature by Sommer et al.
The two-step p-base layer with a gentle concentration gradient in the first step region which is effective to reduce the surface electric field at the edge of junction J.sub.2 has succeeded in making a high-voltage thyristor. This type of thyristor having the two-step concentration profile, however, is still unsuccessful in eliminating the defect due to the high impurity concentration in the vicinity of junction J.sub.3 as mentioned above. In addition, this measure involves the inherent problem raised when creating a large-current thyristor of an average rating current of 250 to 3000 A by extending the diameter of junction surface to about 100 mm. More particularly, in order to achieve the large current capability with a large diameter wafer, it is necessary to uniform the operation within the single crystalline wafer. To this end, a p-base structure accessible to the lateral uniformity is required. In the thyristor having the concentration profile shown in FIG. 2, however, since the gradient of base layer impurity concentration in the vicinity of the junction J.sub.3 between n-emitter and p-base regions (in P.sub.B2 region) is large, even slighr irregularity in dimension, especially in thickness, of the p-base layer due to the manufacturing processes causes the sheet resistance of p-base layer to vary to a great extent. Due to this variation, an irregular spreading of the on-region and a current convergence during turn-off operation occur. Accordingly, this base structure presents difficulties in the manufacture of large-current thyristors having good operational uniformity.
An approach to reduce the high impurity concentration of p-base region at the working point junction is proposed in U.S. Pat. No. 3,990,091 issued Nov. 2, 1976. But this patent aims to decrease the gate sensitivity of the thyristor, and does not satisfactorily solve the problem of operational non-uniformity due to the geometrical irregularity in the thyristor wafer even with the proposed impurity concentration profiles.